1. Field of the Invention
This invention relates to a microwave signal amplifier utilizing transistors, and particularly relates to a circuit configuration of a microwave signal amplifier which has excellent performance in terms of input/output characteristics and stability of single stage or multi-stage amplifiers.
2. Description of the Prior Art
In general, a single stage GH.sub.z band amplifier utilizing a transistor shows a gain inclination (gain vs. frequency characteristic) of -6 dB/octave when the input and output of the said amplifier are matched within the desired frequency band. In the prior art, such a gain inclination within the desired frequency band has been compensated by the following methods: (1) a method of providing frequency characteristic having an adequate attenuation by adequately mismatching (reducing the gain in the low frequency band) the input and the output matching circuits between the stages of a transistor amplifier within the desired frequency band, and (2) a method of matching the input and output matching circuits within the desired frequency band and adding the constant resistance, amplitude-compensating circuit to the discrete amplifier having a gain inclination of -6dB/octave.
However, in the method (1) mentioned above, the mismatch between the transistor and the input/output matching circuits is considerable within the desired frequency band, as well as in the range of desired frequency band; in the method (2) mentioned above, the mismatch between the transistor amplifier and the input/output matching circuits is also considerable in the range outside of the desired frequency band, and such mismatch makes the amplifier unstable. In other words, the circuit is easily liable to oscillate due to thermal noise, or other noise.
In the microwave signal amplifier utilizing a transistor, as described above, the amplifier becomes unstable if the degree of mismatch between the transistor amplifier and the input circuit (generally including the input matching circuit) and output circuit (generally including the output matching circuit), respectively, is large within the desired frequency band. The reason is as follows.
The input reflection coefficient .GAMMA..sub.in and output reflection coefficient .GAMMA..sub.out of the microwave signal amplifier utilizing transistors can be expressed as a function of the S parameters of the discrete transistor as follows: ##EQU1## where S.sub.11 is the input reflection coefficient when the output is terminated without reflection;
S.sub.22 is the output reflection coefficient when the input is terminated without reflection; PA1 S.sub.12 is that transmission coefficient in the reverse direction when the input is terminated without reflection; PA1 S.sub.21 is that transmission coefficient in the forward direction when the output is terminated without reflection; PA1 .GAMMA..sub.S is the reflection coefficient of the input circuit ("power supply" side); and PA1 .GAMMA..sub.L is the reflection coefficient of the output circuit ("load " side).
Here, the input and output reflection coefficients .GAMMA..sub.in and .GAMMA..sub.out of the above-mentioned microwave signal amplifier can be classified, as to their stability or instability, as .vertline..GAMMA..vertline.in, .vertline..GAMMA..vertline.out &lt; 1 (for the stable area), .vertline..GAMMA..vertline.in, .vertline..GAMMA..vertline.out &gt; 1 (for the unstable area). From the above equations, it can be understood that the reflection coefficients .GAMMA..sub.S and .GAMMA..sub.L of the input/output circuits influence the stability of the microwave signal amplifier. Namely, when the matching of the input/output circuits is bad (thus, the absolute values of .GAMMA..sub.S and .GAMMA..sub.L are large), the probability of .vertline..GAMMA..vertline..sub.in or .vertline..GAMMA..vertline..sub.out being greater than 1 becomes large, thus making the amplifier unstable.
In the microwave signal amplifier, when the frequency becomes high, the gain of the transistor decreases (i.e., the transmission coefficient in the forward direction S.sub.21 becomes small), and the probability of the unstable condition of the amplifier is seen to be small within that range of the mismatch frequency higher than the desired frequency band. But, in the mismatch frequency range lower than the desired frequency band, the probability of an unstable condition of the amplifier becomes large due to the fact that, as frequency becomes lower, S.sub.21 of the transistor becomes larger.
In the case of a high power amplifier, such amplifier is often used with a high input level as gain compensate begins. However, when a bipolar transistor (for example, of the common-emitter type) is used, and when the input power exceeds a certain level, an input capacitance between the emitter and the base is pumped by the input signal, and the impedance of the transistor sometimes displays a negative resistance (in this case, .vertline..GAMMA..sub.in &gt; 1.vertline.) for a frequency of 1/2 or 1/3 of the input signal frequency. In such a case, when the impedance of the input matching circuit, in the frequency range of 1/2 or 1/3 of the input signal frequency, is small, .vertline..GAMMA..sub.S .vertline. is large and the condition for oscillation is often satisfied.
In the microwave signal amplifier utilizing a transistor, as described above, when .GAMMA..sub.S, .GAMMA..sub.L (namely, a degree of mismatch between the transistor and the input/output matching circuits) is large, the amplifier often becomes unstable. In the prior art, correction of or compensation for this kind of unstable operation of the amplifier has not been sought.